1. Field of the Invention
This invention relates to hydrocarbon extraction and processing, specifically to heating hydrocarbon-bearing formations in situ for more efficient processing and extraction.
2. Discussion of Prior Art
North American reserves of oil shale and tar sand contain enough hydrocarbonaceous material to be a global provider of hydrocarbons products for the foreseeable future. Large-scale commercial exploitation of certain hydrocarbon-bearing resources, available in huge deposits on the North American continent, has been impeded by a number of problems especially cost of extraction and potentially significant negative environmental impact. Oil shale is also plentiful in the United States, but the cost of useful fuel recovery has been generally noncompetitive. The same is true for tar sands, which occur in estimated vast number of problems, amounts in Western Canada. In addition, heavy or viscous oil is often left untapped in a conventionally-produced oil well, due to the extra cost of extraction. These types of hydrocarbon deposits are becoming increasingly important, as reserves of low viscosity crude petroleum are being quickly depleted.
Materials such as oil shale, tar sands, and coal are amenable to heat processing to produce gases and hydrocarboneous liquids. Generally, the heat develops the porosity, permeability, and/or mobility necessary for recovery. Oil shale is a sedimentary rock, which upon pyrolysis, or distillation, yields a condensable liquid, referred to as a shale oil, and non-condensable gaseous hydrocarbons. The condensable liquid may be refined into products that resemble petroleum products. Oil sand is an erratic mixture of sand, water, and bitumen, with the bitumen typically being present as a film around water-enveloped sand particles. Though difficult, various types of heat processing can release the bitumen, which is an asphalt-like crude oil that is highly viscous.
In the destructive distillation of oil shale or other solid or semi-solid hydrocarbonaceous materials, the solid material is heated to an appropriate temperature and the emitted products are recovered. In practice, however, the limited efficiency of this process has prevented achievement of large-scale commercial application. For example, the desired organic constituent in oil shale, known as kerogen, constitutes a relatively small percentage of the bulk shale material, so very large volumes of shale need to be heated to elevated temperatures in order to yield relatively small amounts of useful end products. The handling of the large amounts of material is, in itself, a problem, as is the disposal of wastes. Also, substantial energy is needed to heat the shale, and the efficiency of the heating process and the need for relatively uniform and rapid heating have been limiting factors on success.
In the case of tar sands, the volume of material to be handled, as compared to the amount of recovered product, is again relatively large, since bitumen typically constitutes only about ten percent of the total, by weight. Material handling of tar sands is particularly difficult even under the best of circumstances. Such processing potentially results in huge, negative environmental impacts.
A number of proposals, broadly classed as in situ methods, have been made for processing and recovering hydrocarbonaceous deposits. Such methods may involve underground heating or retorting of material in place, with little or no mining or disposal of solid material in the formation. Useful constituents of the formation, including heated liquids of reduced viscosity, may be drawn to the surface by a pumping system or forced to the surface by injection techniques. For such methods to be successful, the amount of energy required to effect the extraction must be minimized.
Proposals to use radio frequency to heat relatively large volumes of hydrocarbonaceous formations are exemplified by the disclosures of the following U.S. patents: U.S. Pat. No. 4,140,180 to Bridges et al., 1979; U.S. Pat. No. 4,135,579 to Rowland et al., 1979; U.S. Pat. No. 4,140,179 to Kasevich et al., 1979; U.S. Pat. No. 4,144,935 to Bridges et al., (1980); U.S. Pat. No. 4,193,451 to Dauphine 1980; U.S. Pat. No. 4,457,365 to Kasevich et al., 1984; U.S. Pat. No. 4,470,459 to Copland et al., 1984; U.S. Pat. No. 4,513,815 to Rundell et al., 1985; U.S. Pat. No. 5,109,927 to Supernaw et al., 1992; U.S. Pat. No. 5,236,039 to Edelstein et al., 1993; and U.S. Pat. No. 6,189,611 to Kasevich et al., 2001.
One proposed electrical in situ approach employs a set of arrays of dipole antennas located in a plastic or other dielectric casing in a formation, such as a tar sand formation. A VHF or UHF power source would energize the antennas and cause radiating fields to be emitted into the deposit. However, at these frequencies, and considering the electrical properties of the formations, the field intensity drops rapidly as distance from the antennas increases. Consequently, non-uniform heating results in inefficient overheating of portions of formations in order to obtain at least minimum average heating of the bulk of the formations.
Another past proposal utilizes in situ electrical induction heating of formations. As in other proposals, the process depends on the inherent conduction ability, which is limited even under the best of conditions, of the formations. In particular, secondary induction heating currents are induced in the formations by forming an underground toroidal induction coil and passing electrical current through the turns of the coil. Drilling vertical and horizontal boreholes forms the underground toroid, and conductors are threaded through the boreholes to form the turns of the toroid. However, as the formations are heated and water vapors are removed from it, the formations become more resistive, and greater currents are required to provide the desired heating. In general, the above-mentioned techniques are limited by the relatively low thermal and electrical conductivity of the bulk formations of interest. Thus, the inefficiencies resulting from non-uniform heating render existing techniques slow and inefficient.
Currently, the most commercially accepted method of in situ extraction of hydrocarbons from oil tar sands is the steam flood process that uses a combination of steam or other gaseous pressures along with RF to decrease the viscosity so as to force the oil through the sand to a nearby producer well. This process requires enormous amounts of high-pressure steam that is typically generated with natural gas. On the down side, as price of crude oil increases, the price of natural gas generally rises accordingly, increasing the cost of employing steam flood methods. The steam flood method has been blamed for disrupting natural gas pressures; so the gas producers want to extract their natural gases prior to bitumen recover. But, the users of steam flood bitumen recovery processes need the subterranean pressures from the natural gas reservoirs to assist the steam flood. The loss of the natural gas reservoir can make the steam flood process uneconomical.
Controlled or uniform temperature heating of a hydrocarbonaceous volume to be recovered is desirable, but current methods cannot achieve this goal. Instead, current methods generally result in non-uniform temperature distributions, which can result in the necessity of inefficient overheating of portions of the formations. Extreme temperatures in localized areas may cause damage to the producing volume such as carbonization, skinning of the paraffin waxes, and arcing between the conductors can occur. Furthermore, vaporization of water creates steam that negatively affects the passage of frequency waves to the substances that require heating.
None of the previous proposals for the extraction of hydrocarbons from these types of formations have provided a method of separating the foreign matter from the valuable hydrocarbons prior to extracting to the surface of the earth. The washing of sand from heated oils generally requires steam or other energy consuming processes. The foreign matter in tar sand may contain ten times the desired hydrocarbons. As a result, a substantial negative environmental impact, with respect to disposal of the undesirable foreign matter, would exist if enough hydrocarbons were extracted to support a North American or global demand of oil. Another problem with washing the sand from the oil is the amount of water that would be required for large-scale production. Not only would tremendous amounts of fresh water be required, but also disposal of the resulting contaminated water would be an important issue. Disposing of the undesirable organic and inorganic substances such as heavy metals, sulfur, etc that would be separated from the hydrocarbons would impose additional environmental challenges. Furthermore, extracting large amounts of heated bitumen and heavy oils to the surface of the earth can release sizable amounts of greenhouse gases and other pollutants into the atmosphere during the ensuing washing, crude storage, separating, and refining processes.
Although RF dielectric heating systems have been used for heating hydrocarbon-bearing formations in the past, there remains a need for improved apparatuses and process techniques to rapidly, efficiently, and uniformly heat specific chemical compositions that reside in bitumen, and/or individual hydrocarbon compositions. There also is a substantial need for a method of separating the undesirable matter from the hydrocarbons and leaving it generally disposed in the context of its original environment.
Disadvantages of Capacitive RF Dielectric Heating
A specific disadvantage of known capacitive RF dielectric heating methods is the potential for thermal runaway or hot spots in a heterogeneous medium since the dielectric losses are often strong functions of temperature. Another disadvantage of capacitive heating is the potential for dielectric breakdown (arcing) if the electric field strengths are too high across the sample. Thicker samples with fewer air gaps allow operation at a lower voltage.
Prior Art
FIGS. 1–4 (Prior Art) show an example of a known capacitive RF dielectric heating system. A high voltage RF frequency sinusoidal AC signal is applied to a set of parallel electrodes 20 and 22 on opposite sides of a dielectric medium 24. Medium 24 to be heated is located between electrodes 20 and 22, in an area defined as the product treatment zone. An AC displacement current flows through medium 24 as a result of polar molecules in the medium aligning and rotating in opposite fashion to the applied AC electric field. Direct conduction does not occur. Instead, an effective AC current flows through the capacitor due to polar molecules with effective charges rotating back and forth. Heating occurs because these polar molecules encounter interactions with neighboring molecules, resulting in lattice and frictional losses as they rotate.
The resultant electrical equivalent circuit of the device of FIG. 1 is therefore a capacitor in parallel with a resistor, as shown in FIG. 2A. There is an in-phase IR component and an out-of-phase IC component of the current, relative to the applied RF voltage. In-phase component IR corresponds to the resistive voltage loss. These losses get higher as the frequency of the applied signal is increased for a fixed electric field intensity or voltage gradient due to higher speed interactions with the neighboring molecules. The higher the frequency of the alternating field, the greater the energy imparted into medium 24 until the frequency is so high that the rotating molecules can no longer keep up with the external field due to lattice limitations.
This frequency, which is referred to as a “Debye resonance frequency” after the mathematician who modeled it, represents the frequency at which lattice limitations occur. Debye resonance frequency is the frequency at which the maximum energy can be imparted into a medium for a given electric field strength (and therefore the maximum heating). This high frequency limitation is inversely proportional to the complexity of the polar molecule. For example, hydrocarbons with polar side groups or chains have a slower rotation limitation, and thus lower Debye resonance, than simple polar water molecules. These Debye resonance frequencies also shift with temperature as the medium 24 is heated.
FIGS. 2A, 2B, and 2C are equivalent circuit diagrams of the dielectric heating system of FIG. 1 for different types of hydrocarbon-bearing formations. Resultant electrical equivalent circuits may be different from the circuit shown in FIG. 2A, depending on the medium 24. For example, in a medium 24 such as a hydrocarbonaceous formation with a high moisture and salt content, the electrical circuit only requires a resistor (FIG. 2B), because the ohmic properties dominate. For media with low salinity and moisture, however, the resultant electrical circuit is a capacitor in series with a resistor (FIG. 2C).
Various other hydrocarbons, elements, or compositions within a hydrocarbon-bearing formation may use different electrical circuit analogs. More complex models having serial and parallel aspects in combination to address second order effects are possible. Any of the components in any of the models may have temperature and frequency dependence.
An example of a conventional RF heating system is shown in FIGS. 3 and 4 (Prior Art). In this system, a high voltage transformer/rectifier combination provides a high-rectified positive voltage (5 kV to 15 kV) to the anode of a standard triode power oscillator tube. A tuned circuit (parallel inductor and capacitor tank circuit) is connected between the anode and grounded cathode of such tube as shown in FIG. 4, and also is part of a positive feedback circuit inductively coupled from the cathode to the grid of the tube to enable oscillation thereby generating the RF signal. This RF signal generator circuit output then goes to the combined capacitive dielectric and resistive/ohmic heating load through an adapter network consisting of a coupling circuit and a matching system to match the impedance of the load and maximize heating power delivery to the load, as shown in FIG. 3. An applicator includes an electrode system that delivers the RF energy to the medium 24 to be heated, as shown in FIG. 1.
The known system of FIGS. 1–4 can only operate over a narrow band and only at a fixed frequency, typically as specified by existing ISM (Industrial, Scientific, Medical) bands. Such a narrow operating band does not allow for tuning of the impedance. Any adjustment to the system parameters must be made manually and while the system is not operating. Also, the selected frequency can drift. Therefore, to the extent that the known system provides any control, such control is not precise, robust, real time or automatic.